Selective echo suppressor



Nov. 6, 1928. 1,690,153

L. G. ABRAHAM SELECTIVE ECHO SUPPRES SOR Filed May 26, 1926 IN VENTO/1 3echo eects become serious.

41)k through one path while only reected currentsor echoes pass throughthe other. It

Patented Nov. 6, 1928.

UNITED STATES momma: o. naman, or BaooxLYN, NEW Yeux, AssimNoR ToAMERICAN TELE- PATENT OFFICE.

PHONE AND TELEGRAPH COMPANY, A. CORPORATION OFNEW YORK.

y sELncTrvE ECHO surrnnsson.

y Application mea may 26, 192s. seran 12?. 111,847.

This invention relates to transmission systems, andparticularly toimprovements in devices for suppressing echo effects thereon. Primarilythe object of the invention 5 fonsists in providing a type of echosuppressor which will enable the receiving station to,transmit in theopposite direction with certain frequencies at the same time thatreception is going o n withother frequencies.

As is well known`VA in the art in. designing telephone circuits whichare electrically long, an important. problem` is presented by thenecessity of avoiding serious echo eiects. Echo e ects are caused byreflections of eleci trical `waves which takeA place whenever electricalirregularities are encountered in telephone circuits. The effectsproduced are very ysimilar'to echoes of sound waves. Some of thereilected` waves return/ to the receiver of 2 the'talkers telephone sothat if the eects are severe, he may lhear an echo of his own words.Other reflected waves enter the receiver of the listeners telephone and,if severe,zcause the listener to hear an echo fol- 125 lowing thedirectly received transmission.

While reflections of voice waves may occur in alll practical telephonecircuits, it is only in telephone circuits of such length as to requirea number of repeaters, however, that The fact that the circuits areelectrically verylong makes the time lag ofthe echoes appreciable. Atthe same time the telephone repeaters overcome the high attenuation ofthese long cir- `cuits, and,y consequently, make the echoes louder. -Inall practical telephone circuits involving more than a single repeater,there are points where the transmission in the two ,Y

directions paes through two separate paths. At these points the directtransmission passes has beenL the practice to`locate the echo suppressorat one of these points. In this device.

4 which is well known in the m, the electrical currents, with the helpof vacuum tubes, are

' caused to actuate relays which cut o' theA quencies transmission. 4Fora full and complete discussion and illustration of echo sup essors,reference is made, to a paper by A. Clark and R. C. Mathes, entitledEcho Suppressors for long telephone circuits, published June, 1925,inthe Journal of the American Institute of Electrical Engineers, Vol.XLIV, pp.

It is pointed out that it has been the practice heretofore for the echosuppressor devices in cuttingoif the echoes in the return path to closea short clrcuit across said ref turn path. Obviouslythis would .preventthe receiving station from any type of sending during reception.Instances might arise, however, in which it would be desirable to makeit possible for the receiving station to signal during reception at someother fre- -i .of this invention is so arranged that it will not placea'complete short circuit across the return path, but will merely shortcircuit or close the return path to certain frequency bands while stilla-llowin other bands to be transmitted.' According? echo effects fromthe original frequency band will be obviated without completely closingtransmission in the return ath. In'other words, the invention provi esan .echo suppressor with selective properties with respect to di'erentfre- Oth'er objects and features of the invention willappear more fullyfrom the detailed description thereof` hereinafter'4 given.

The invention may bev more fully understood from the followingdescription, to-

gether with the accompanying drawing, in

the Figures 1, 2 and 3 of which are shown circuit d1a-grams illustratinga preferred form of the invention and/modifications thereof, Similarreference characters have been utilized to denote like parts in all ofthe figures In Fig. 1 are shown two line sections L1 and L2interconnected by two one-way circuits including the repeater, oramplifier, elements 1 and 4. Lines L1 and L2 are connected to thethrcc-winding transformers 2 and 3 and the balancing networks N, and N2,respectively, in a manner well known in the art. Bridged across theoutput circuit of amplifier 1 is a circuit including anamplifier-detector 5 and a relay 7. This relay 7 will respond to thecurrents transmitted through the amplifier 1 and upon operating, willclose the circuit 9. In former types of echo Suppressors the closing ofthe circuit 9 would close a dead short circuit across the output ofamplier 4 and hence stop all transmission from line L2 to line L1.However, in the arrangements of this invention a combination 12 ofinductance and capacity is placed in circuit 9 so that it will beanti-resonant at some desirable signaling frei,

quency such as 765 cycles. Accordingly, when circuit 9 is closed,it'will place a shunt across the output of repeater 4 which will preventthe transmission of echo currents of the frequency band transmittedthrough ainplifier 1, but will allow some other signaling frequency orfrequencies, such as 765 cycles, to be transmitted without appreciableattenuation from line L2 to line L1. To prevent echoes when transmissionis taking place from line L2 to line L1, a circuit is bridged across theout ut of amplifier 4 which includes an amp ifier-detector 6 and arelay8. The relay 8 controls a. circuit 10, which is similar to circuit 9.Circuit 10 includes a combination 11 of inductance and capacity, wherebythe circuit 10 will be anti-resonant at some desirable signalingfrequency, such as 765 cycles. Accordingly, it will prevent echoes frombeing transmitted through arnplifier 1 but will at the same time allowsignaling at a fre uency of 765 cycles to take place from line L, toline L2. The amplifierdetectors are assumed to be adjusted by meanswell-known in the art so that they will have their greatest sensitivityto currents of frequencies betweenl 900 and 17 00 cycles, this being,for example, the transmission band utilized for picture work, and are soinsensitive to currents of 765 cycles that they will not cause operationof the echo-suppressor relays. n

In Fig. 2 is shown a modification of the arrangements of Fig. 1. Similarreference characters have been utilized to denote like parts in botharrangements. The only way in which the arrangements differ is in theconstruction of the shunt circuits 9 and 10. Instead of making thesecircuits antiresonant at thesignaling frequency of 765 cyclestransmitted from line L2 to line L2,

these shunt circuits are made resonant at the central frequency of aband transmitted from line L, to line L2, such as 900 to 1700. This isaccomplished by the adjustment of the inductances 11, 11', thecondensers 12, 12', and the resistanccs 13 and 13 in a manner well knownin the art. Accordingly, these shunts when closed will prevent echoesfrom'the frequency band, such as of 900 to 1700 cycles, but will notinterfere with signaling frequencies some distance outside of this rangetransmitted in the opposite direction. As the arrangements are in otherrespects similar te those of Fig. 1, no further description thereof willbe given.

In Fig. 3 is shown another modification of the echo suppressor of theinvention in which like reference characters have been utilized todenote like parts with respect to Figs. 1 and 2. The arrangements ofFig. 3 differ from those heretofore described in that the shunt circuits9 and 10 are omitted and the relays 7 and S are utilized to insert inthe output of the repeaters low pass. high pass, or other filtcrs,suchas F1 and F2. which will cut off the echo currents but will allow thetransmission of signaling currents of certain frequencies. In otherwords, the original signaling band transmitted through amplifier 1 willoperate relay 7 which will insert in the output of amplifier 4 the lowpass filter F2. rlhis will prevent echoes of the original signal bandfrom being repeated back to line L2 but will allow lower frequencysignals to be ltransmitted from line L2 to line L,. Relay 8 and filterF1 operate in a similar manner.

It is pointed out that the frequency values of a band from 900 to 1700and 765 cycles are utilized for purposes of illustration only and thatthe invention extends outside of any such specific limits. Accordingly,while the invention has been disclosed as embodied in certain specificarrangements which are deemed' desirable, it is understood that itiscapable of embodiment in many other widely varied forms withoutdeparting from the spirit of the invention as defined by the appendedclaims.

Vhat is claimed is:

1. A circuit including an echo suppressor, and means in said echosuppressor providing for selective operation thereof with respect todifferent frequencies transmitted over said l way line sections fortransmission in opposite directions, amplifiers in each of said 11nesections, a. circuit bridged across one of said line sections on theoutput side of the am- `pliier therein, said circuit including an ingmeans whereby it will be anti-resonant at 10 a certain frequency, andmeans controlled by said relay means for opening and closing said lastmentioned circuit.

In testimony whereof, I have signed myname to this specification this24th day of 15 May, 1926.

LEONARD G. ABRAHAM.

